1. Field of the Invention (Technical Field)
The present invention relates to apparatuses and method for optical dithering.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
An increasingly popular technique in ultraviolet (“UV”), optical, and infrared (“IR”) imaging observations (including Forward Looking Infrared Radar (“FLIR”) applications) involves the use of “dithering,” or spatially offsetting a telescope or sensor by shifts that are generally small relative to the detector size, thereby moving the target to a number of different locations on the detector. Two of the main strategies involve offsets by integer pixels to facilitate the removal of bad pixels, and offsets by sub-integer pixels to improve spatial sampling of a point spread function (PSF).
The benefits of dithering include: (1) Dithering can reduce the effects of pixel-to-pixel errors in the flat-field or spatially varying detector sensitivity; (2) Integer shifts of a few pixels allow the removal of small scale detector defects such as hot pixels, bad columns, and charge traps from the image; and (3) Non-integral (sub-pixel) dithers allow the recovery of some of the information lost to undersampling by pixels that are not small compared to a point spread function. However, significant alignment overhead is associated with dithering, particularly in telescopic applications.
Dithering in the context of telescope observations is discussed in detail in A. M. Koekemoer, et al., HST Dither Handbook Ver. 2.0, chs. 1-3 (Baltimore, Space Telescope Science Institute, 2002). Mechanical dithering devices include BEI Precision Systems and Space Division's Model SPC40-030A1 Dither Optical Scanner. The present invention is of a low-cost, refractive, two axes mechanical dithering apparatus.
U.S. Pat. No. 4,961,627, entitled “Two-Axis Beam Steering Apparatus”, to Swain et al., is directed to a device for steering a light beam which requires a window having a first flat side and a second convex side. The device also requires a secondary window, which is rotatable with respect to the first window, having a concave first side and a flat second side. The device of Swain et al. further requires that the first and second windows be spaced apart by a “desired gap” (Col. 1, line 31). Still further, the device of Swain et al. requires that a flexible seal be provided and that a fluid be disposed between the two windows. As such, the device disclosed by Swain et al. is not only intricate and costly to manufacture, but is also susceptible to leaking which renders the device inoperable.
U.S. Pat. No. 6,332,576, entitled “Dithering Assemblies for Barcode Scanners”, to Colley et al. is directed to a device for dithering a laser beam, particularly those used for barcode scanners. The dithering device of Colley et al. uses a feedback system. However, Colley et al. disclose that the device can be “constructed without employing feedback to resonantly drive the dithering assembly provided that the drive force is sufficiently large to produce the maximum amplitude allowed by the travel stops under non-resonant conditions” (Col. 7, lines 37-41). The device of Colley et al. employs a movable mirror “for scanning an illumination beam of a barcode scanner” (Lines 2 and 3 of the Colley et al. Abstract). As such, the angle of the beam is shifted rather than simply linearly displaced. Further, the disclosure of Colley et al. is directed to moving a beam across a large span (e.g. the width of a bar code scan).
There is thus a present need for a method and apparatus which enables rapid linear displacement across a short distance, (e.g. ½ the distance between adjacent pixels in an optical array), without the added complexity of a feedback system.